Electrochemical Cell Stack

ABSTRACT

The present invention relates to a cell stack having at least one electrochemical cell arranged between a first end plate connected to an electrical bolt and a second end plate connected to another electrical bolt, said stack having a housing, means for providing fixed support of said stack to said housing and means for maintaining a constant load over said cell stack. Said means for maintaining a constant load comprise an elastic pad inserted into the space between said cell stack and said housing wall. The cell stack is applicable in the area of PEM fuel cells and PEM water electrolyser cells.

The present invention relates to an electrochemical cell stack havingmeans for maintaining constant mechanical load over the components insaid cell stack.

Electrochemical cells using a polymer membrane as electrolyte, e.g. aProton Exchange Membrane (PEM), are interesting in the area of PEM fuelcells and PEM water electrolyser cells where protons constitute thecarrier of ionic charge.

Each cell consists of an assembly of a membrane and two catalytic layerson each side, i.e. a Membrane Electrode Assembly (MEA) where thecatalytic layers constitute the electrodes and are in intimate contactwith the membrane. The MEA is further supported by electrode backings oneach side, typically a porous plate, a mesh or a porous sheet with goodelectrically conductivity and with high electrochemical/chemicalstability. The electrode backing has a certain integrity which matchesthe electrode layer and the operation regime. The water electrolysisprocess in a PEM cell is shown in FIG. 1.

PEM cells, and similar cells, are in most practical applicationsarranged in a so-called bipolar design, i.e. the cells are connected inseries, in a cell stack, where the current passes from one cell toanother. Each cell is enclosed within so-called bipolar plates or endplates, which separate the individual PEM cells from each other andwhere electronically current passes from cell to cell. Between thebipolar plate and the electrode backing there must be some void spacefor transport of fluid in and out of the cell and at the same time theremust be electrical contact between the bipolar plate (or end plate) andthe electrode backing (as shown in FIGS. 2 a and 2 b).

Large stack units, with a considerable number of cells in series,provide many interfaces between all the layers, where every interfacepossesses an interfacial contact resistance to passage of electricalcurrent. Different materials possess different interfacial contactresistance. It is therefore of great importance to secure highelectronically contact between all the different interfaces within acell assembly to minimize the electrical contact resistance.

The electrical contact resistance represents a direct energy loss givenby:

P=I ² ·R

where P is the energy loss in watt, I is the electronic current inampere and R is the contact resistance in ohm. The energy loss istransformed to heat and, in the case where metallic components are usedwithin the cells, local heating further accelerates corrosion andpassivation of the metallic interfaces, which further increases theohmic resistance and energy loss.

The contact resistance is a function of the mechanical load thatcompresses the cell together, i.e. the compression force or the clampingforce. According to classical theory the contact resistance as afunction of the compression force (F) is given by:

R=k·F ^(−x)

where k is a constant, F is the compression force and x is a constantdepending on the type of deformation at the contact point and is near0.5 for most contacts. From this equation it is clear that R decreasessubstantially until a certain load F where R decreases onlyinfinitesimal for higher F.

During the operation of a PEM cell, i.e. an electrolyser or a fuel cell,heat is generated. At a given temperature the water content of themembranes is in equilibrium with the surrounding environment. Increasedwater content causes swelling of the membrane whereas decreased watercontent causes shrinking of the membrane. The dimensions of the othercell parts will also vary with temperature. During operation, each celltherefore expands and contracts according to the temperature caused bythe heat generated during operation. Under repeated expansion andcontraction of the cell, the cell parts that support the electrodes,e.g. gas diffusion layers in fuel cells, current/water/gas distributionlayers and electrode supports, will undergo mainly elastic deformation.However, in the process of expansion and contraction some constituentsof a cell stack will be deformed plastically to a certain degree.

In order to ensure gas tightness of the cell, when operating thecell/cell stack under pressure, it is often beneficial to insert thecell body into a pressurised housing (a pressure vessel), where theelectrochemical cell is in open communication, or at least partly, withthe interior of the housing. Still a compression force over the endplates of the cell body is necessary.

Either the anode side or the cathode side must be in open communicationwith the vessel environment surrounding the cell stack, and the pressuredifference between anode and cathode side must then be controlledexternally. Depending on the specific cell design the MEA can withstandseveral bars pressure difference.

For an electrochemical cell stack the clamping pressure over the cellstack is important for maintaining electronic contact between thestacked components. Due to expansion and contraction of the cell stackduring operation, and thereby relaxation of the clamping force, a meansis necessary to maintain almost constant mechanical load over the cellstack.

Means for maintaining constant load is well known in the art of PEMtechnology and is absolutely necessary to obtain long operation lifetimeof the electrochemical cell. Different concepts have been used indifferent designs to provide some beneficial features. Low costcomponents are the most important but also more advance components withextra functionality or advantages can be provided.

The conventional means clamp the cell stack between two end plates byseveral bolts and using springs to bear the load as shown in FIG. 3.

For high-pressure systems, where the cell stack is inserted into apressurised housing, the conventional spring system is not suitable asthe springs are usually made from materials that will lead to poisoningof the environment within the housing.

GB 497956 describes a very simple system where springs are insertedbetween a press plate and a first cell body end plate. Tie rods arepressing the second cell body end plate and the press plate together.

EP 1231298-A1 describes another simple and a very common method. The tierods, or bolts, compress the cell body end plates together using springsbetween the bolt nuts and one cell body end plate. A similar design isalso shown in WO 0209208-A2.

JP 2003-160891-A describes a more advanced system using a hydrauliccylinder between a press plate and one cell body end plate to maintainconstant compression over the cell body when the cell is operating undervarying pressure.

U.S. Pat. No. 3,507,704 describes a so-called regenerative alkaline fuelcell system, i.e. an electrochemical device that can operate both as anelectrolyser and a fuel cell, where the cell body is inserted into atank. The cell body is compressed by tie rods and springs between thebolt nut and one cell body end plate. Material compatibility of tie rodsand springs with the chemical environment within the interior of thevessel and within the cell stack is very important.

JP 2003-160891-A describes a system where the oxygen pressure is used tocompress the cell stack by pressing a moveable electrical end platetowards the opposite electrical end plate.

Several patents, e.g. U.S. Pat. No. 5,547,777 and US 20040115511 A1,describe a thin film compression layer located at the end plate of thecell body, or within the compartment of each cell, for evening outlocally pressure differences within the cells. These may be caused forexample by an uneven thickness of different electrically conducting cellparts or by bending of the cell end plates during compression. The mainpurpose of these thin film compression layers is to secure a highinter-layer contact area, i.e. of the area of the electricallyconductive layers. Also, the thin film compression layer is locatedwithin one or more cell compartments and compensates only within thegiven cell.

EP 1304757-A2 describes an electrochemical cell body compressed betweentwo plates. Since the plates become bent under compression of the cellstack, two electrical conductive elements, that undergo temporaryplastic deformation, are inserted between each of the compression plateand each of the two cell body plates. Upon compressing the cell stack,the plastic deformation of the two elements secures high contact surfacebetween the bent compression plates and the cell body end plates.

The main objective of the present invention was to arrive at anelectrochemical cell stack which is designed to maintain an almostconstant load over said cell stack in order to secure long termoperability of said cell stack.

Another objective of the present invention was to arrive at anelectrochemical cell stack which is designed to compensate thermalexpansion or contraction of components of said cell stack duringoperation.

A further objective of the present invention was to arrive at anelectrochemical cell stack which is designed to secure proper sealingaround electrical bolts and insulating the electrical end plate(s) fromits environment.

Still another objective of the present invention was to arrive at anelectrochemical cell stack which is designed to avoid poisoning of itsenvironment.

The environment is the void space inside a housing where the cell stackis inserted.

In accordance with the present invention, these objectives areaccomplished by a cell stack having at least one electrochemical cellarranged between a first end plate connected to an electrical bolt and asecond end plate connected to another electrical bolt, said stack havinga housing, means for providing fixed support of said cell stack to saidhousing and means for maintaining a constant mechanical load over saidcell stack. Said means for maintaining a constant load comprise at leastone elastic pad inserted into the space between said cell stack and saidhousing wall.

Preferably, said pad is inserted into the space between one of said endplates and the adjacent end plate of said housing.

The cell stack can also have an elastic pad in each end. The pad is theninserted into the space between each of said end plates and each of itsadjacent end plate of said housing.

An elastic pad means that the pad has the ability to recover itsoriginal shape partially or completely after the deforming force(thermal expansion or contraction) has been removed.

Preferably, said pad is made of silicon or another elastic polymericmaterial. Its shape can be spherical or it can be a pad with indentationon the side, i.e. like a bellow.

Alternatively, the elastic pad can be in the form of two or moreindividual pads.

The pad is placed on at least one of the end plates of the cell stack.

When in operation, the cell stack expands or contracts. This movement iscountered by the compression and expansion of the pad thereby providingthe necessary pressure on the cell stack at any time.

Thus, the pressure acting on the several parts of the cell stack will besubstantially constant and always so high that perfect tightness issecured. This will secure a long-term operability of said cell stack.

The thickness of the pad is chosen such that its elastic properties areequivalent to those of the cell stack. Thereby any thermal elongation orcontraction of the cell stack under operation will be compensated. Theelastic pad secures a high contact surface throughout operation of thecell. Furthermore, the elastomeric compression pad is made of a materialthat is viscoelastic, and will undergo plastic deformation over time.Therefore the thickness of the pad must be much larger than the expectedplastic deformation, in order to maintain a minimum load over the cellstack.

The use of an elastic pad has the advantage of bringing a compressionpad into the housing without compromising the materials compatibility.Furthermore, said pad is electrical insulating and serves the purpose ofisolating the bolt for electrical connection. Furthermore, the sealingproperties of the elastic pad offer the option of bringing current-boltsof high electronic conductivity but with a poisoning-effect into thehousing.

The pad functions both as a spring and as a sealing. Hence, theconventional used springs, nuts and bolts are replaced with said pad.

In one embodiment of the present invention the housing can be a pressurevessel.

The present invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 shows a membrane electrode assembly (MEA),

FIGS. 2 a and 2 b show possible concepts of bipolar plates and theintegration of flow fields in a cell,

FIG. 3 shows a cell stack clamped between two end plates usingconventional dish-springs to maintain a constant load,

FIG. 4 shows a cell stack inside a housing according to the presentinvention.

FIG. 1 illustrates a water electrolysis process in a PEM cell wherewater is fed to the anode side 1 and, under applied potential field,becomes split to oxygen gas, protons and electrons. Protons migrate tothe cathode side 3 where it recombines with an electron to form hydrogengas.

FIGS. 2 a and 2 b show two typical concepts where a) the flow pattern isintegrated into the bipolar plate or b) a porous layer is insertedbetween the bipolar plate and the electrode backing. This porous layermust then provide electronic contact between the bipolar plate and theelectrode backing and it must provide flow of fluid in and out of thecell compartment. Gaskets are placed around the electrode backing areabetween the membrane and the bipolar plate (not shown in FIG. 2). Flowof fluid in and out of the cells is typically arranged through channelswithin the gaskets and the bipolar plates.

FIG. 3 shows a cell stack 4 of 10 cells connected in a bipolararrangement, where the cells are clamped between a first end plate 1 anda second end plate 8. Number of cells can be from one to severalhundred. The end plates are compressed together by a certain number ofbolts 2, usually more than four bolts. Springs 5 are inserted on thebolts between the second end plate 8 and the nut 7 to maintain an almostconstant clamping pressure.

FIG. 4 shows a housing comprising a wall 5 and a first end plate 2 and asecond end plate 8. An electrochemical cell stack is compressed betweenend plates 2 and 8. The end plate 2 functions also as an electrical endplate and is connected to an electrical end bolt 1. The electrochemicalcell stack 3 is arranged between said electrical end plate 2 and asecond electrical end plate 6. The electrical end plate 6 is connectedto an electrical bolt 10 and inserted into the housing and electricallyinsulated from the housing. The insulation is achieved by an insulationring 9 between bolt 10 and end plate 8 and an electrical insulatingelastic pad 7. The elastic pad 7 is placed between the electrical endplate 6 and end plate 8.

The electrochemical cell stack is stacked on the top of the end plate 6,the end plate 6 is placed on the pad 7 and the end plate 8. Afterstacking, the wall 5 is placed on and fixed to the plate 8. End plate 2is then positioned on the top of the cell stack. By exerting a forcenormal to end plate 2 the stack is compressed until sealing is obtainedbetween the end plate 2 and the wall 5 and the end plate 8. End plate 2and end plate 8 are fixed to the wall 5.

Said elastic pad has the following distinct functions:

-   -   to insulate the electrical end plate (6) from the housing.    -   to assure sealing around electrical bolt (9).    -   to exert the correct compression force on the end plate of the        cell stack.    -   to compensate thermal expansion or contraction of the cell stack        under operation while maintaining almost constant compression        force.

Having described a preferred embodiment of the present invention it willbe apparent to those skilled in the art that other embodimentsincorporating the concepts may be used. The embodiments illustratedabove are intended by way of example only and the actual scope of thepresent invention is to be determined from the following claims.

1. A cell stack having at least one electrochemical cell arrangedbetween a first end plate connected to an electrical bolt and a secondend plate connected to another electrical bolt, where said stack havinga housing, means for providing fixed support of said cell stack to saidhousing and means for maintaining a constant mechanical load over saidcell stack, characterised in that said means for maintaining a constantload comprise at least one elastic pad inserted into the space betweensaid cell stack and said housing wall.
 2. A cell stack according toclaim 1, characterised in that said pad is inserted into the spacebetween one of said end plates and the adjacent end plate of saidhousing.
 3. A cell stack according to claim 1, characterised in thatsaid pad is inserted into the space between each of said end plates andeach of its adjacent end plate of said housing.
 4. A cell stackaccording to claim 1, characterised in that said means for maintaining aconstant load comprise two or more individual pads.
 5. A cell stackaccording to claim 1, characterised in that said elastic pad is asilicon pad.
 6. A cell stack according to claim 1, characterised in thatsaid pad is spherical shaped.
 7. A cell stack according to claim 1,characterised in that said pad has indentation.
 8. A cell stackaccording to claim 1, characterised in that said housing is a pressurevessel.